56 
J. T. ELSDEN—MICROSCOPIC STRUCTURE 
localities which appear most likely to repay an actual comparison. 
IN’o one, having read Mr. Allport’s description of the British 
Carboniferous dolerites,*' can fail to recognise the very great 
similarity between them and the doleritic boulders of Hertfordshire ; 
and, although complete identification from written descriptions 
would be impossible, there are some considerations which lead 
me to the conclusion that it is amongst the Scotch dolerites, 
rather than those of the Midlands, that we may expect to find the 
counterparts of the rocks under present discussion. There appear 
to be certain features which are more especially characteristic 
of the Scotch specimens examined by Mr. Allport, and these are 
the presence of biotite , the occurrence of quartz as a secondary 
constituent, the presence of olivine partially converted into hema¬ 
tite , and the common occurrence of serpentinous and chloritic 
olivine pseudomorphs. How these characters have one and all 
been noticed in several of the specimens described above, and, 
in addition, it may be mentioned that the chloritic-felspar pseudo¬ 
morphs, observed in the specimen labelled Ashivell, 123, are also 
characteristic of several Scotch dolerites, notably, as far as my 
own specimens are concerned, in samples from Airdrie and Burnt 
Island. "Without, therefore, venturing to assert that the dolerites 
are of Scotch origin, I think there is, at least, sufficient evidence to 
render comparison advisable with the Scotch rocks before those of 
the Midland counties. 
Turning now to the porphyrites, I know of no localities nearer 
than the Cheviot Hills whence these rocks could have been derived; 
but here again I am speaking on the authority of written descrip¬ 
tions, and not of actual comparison. On reading Mr. Harris Teall’s 
paper “On the Cheviot Andesites and Porphyrites,” f with these 
Hertfordshire porphyrites before me, I could scarcely believe that 
the specimens were not identical. A comparison should certainly 
be made in this case, and I have little doubt but that the original 
abode of these boulders will be found to be either in the Cheviots, 
or amongst the hill-ranges of South Scotland. 
The granitic and gneissose rocks present considerably more 
difficulty, although perhaps the localities of British granites are 
far better known than those of other igneous rocks. The specimen 
labelled Ashwell, 72, bears a strong resemblance to a section in my 
cabinet from Dalbeattie’s quarries, Kirkcudbright, and the specimen 
labelled Sandon , 3, might be a finer-grained variety of one of the 
so-called syenites from Charnwood Forest. Other possible localities 
are the English Lake District and the Grampians, of which, judging 
from the other specimens, the latter would be the more probable. 
It must not be forgotten, however, that during the Glacial epoch 
an ice-sheet probably extended over what is now the German Ocean, 
from Scandinavia to our eastern shores, and it is not impossible that 
some of the Hertfordshire boulders may have had a more distant 
source than the limits of our own islands. 
* ‘ Quart. Journ. Geol. Soc.,’ vol. xxx, p. 529 et seq. 
f ‘ Geol. Mag.,’ decade ii, vol. x, pp. 97, 145, 252. 
